1. Field Of The Invention
This invention relates to methods for stimulating subterranean formations utilizing viscous fluids to enhance production of formation fluids to enhance production of formation fluids. More particularly, the invention relates to methods of increasing the viscosity of a viscous fluid at elevated temperatures to facilitate stimulation of a subterranean formation.
2. Description Of The Prior Art
Hydraulic fracturing is a widely used technique for stimulating the production of fluids, such as petroleum, from subterranean formations. Fracturing is commonly performed by contacting a subterranean formation with a viscous fracturing fluid that may contain particulate solids, hereinafter referred to as propping agents or proppant suspended therein. Sufficient pressure is applied to the fracturing fluid by surface pumping equipment to cause the opening of a fracture in the subterranean formation by the transmitted pressure. The elevated pressure is maintained on the fluid for a sufficient time to cause the fracture to extend into the subterranean formation by movement of the fluid into the fracture. When the pressure on the fracturing fluid is reduced, the propping agents in the fluid prevent the complete closure of the created fracture.
Viscous fluids are desirably used as fracturing fluids because they have been found to remain in the fractures, through build-up of a filter cake of the fracture faces, for a sufficient time to permit satisfactory extension of a fracture into a subterranean formation. Additionally a viscous fracturing fluid facilitates the transport of a propping agent into the created fracture.
A significant problem is encountered where the bottom hole temperature or the temperature in the vicinity or locus of the fracture exceeds about 175.degree. F.-200.degree. F. At such elevated temperatures, many types of viscous fluids undergo a reduction in viscosity and therefore a reduction in their fracturing capability.
It would be desirable to provide a viscous fluid that is capable of readily transporting proppant at ambient temperatures yet develops sufficient viscosity to satisfactorily fracture a formation having a temperature in excess of about 175.degree. F.